As a method for producing a silicon single crystal used in producing a semiconductor device, a CZ (Czochralski) method, in which a silicon single crystal is pulled out of a raw material melt in a quartz crucible while being grown, is widely employed. In a CZ method, a seed crystal is immersed in a raw material melt (a silicon melt) in a quartz crucible under an inert gas atmosphere and a silicon single crystal having a desired diameter is grown by pulling the seed crystal while rotating the quartz crucible and the seed crystal.
With the advancement of high integration and accompanying miniaturization of semiconductor devices, growth defects in a silicon wafer (grown-in defects) have recently been a problem. Crystal defects result in degrading the characteristics of semiconductor devices and is more and more influential as the miniaturization of the device advances. As such growth defects, octahedral void-like defects as a cluster of vacancies in a silicon single crystal produced by a CZ method (Analysis of side-wall structure of grown-in twin-type octahedral defects in Czochralski silicon, Jpn. J. Appl. Phys. Vol. 37 (1998) pp. 1667-1670), a dislocation cluster formed as a cluster of interstitial silicon (Evaluation of microdefects in as-grown silicon crystals, Mat. Res. Soc. Symp. Proc. Vol. 262 (1992) pp. 51-56) and the like are known.
It is indicated that the introduced amount of these kinds of growth defects is determined by a temperature gradient of a crystal at a growth interface and a growth rate of a silicon single crystal (The mechanism of swirl defects formation in silicon, Journal of Crystal growth, 1982, pp. 625-643). Regarding a method for producing a low-defect silicon single crystal using this, Japanese Patent Application Laid-Open Publication No. H6-56588, for example, discloses reducing the growth rate of a silicon single crystal and Japanese Patent Application Laid-Open Publication No. H7-257991 discloses pulling a silicon single crystal at a rate not exceeding the maximum pulling rate approximately proportional to the temperature gradient in a solid-liquid interface area of a silicon single crystal. In addition, an improved CZ method, which focuses on a temperature gradient (G) and a growth rate (V) during the crystal growth or the like, is also reported (The Japanese Association for Crystal Growth Cooperation vol. 25 No. 5, 1998) and it is necessary to control the crystal temperature gradient with high precision.
In these methods, in order to control the crystal temperature gradient, there is provided a cylinder-shaped or inverted cone-shaped structure for insulating the radiant heat (a heat insulating member) above the melt surface around the silicon single crystal to be grown. Because this steepens the crystal temperature gradient at a high temperature of a crystal, these methods have the advantage of obtaining a defect-free crystal at high speed. However, in order to control the crystal temperature gradient accurately, it is necessary to control with very high precision the distance between the melt surface and the heat insulating member located above the melt surface in such a manner that a predetermined distance is maintained. However, it has been difficult to control with precision the distance between the melt surface and the heat insulating member in such a manner that the predetermined distance is maintained.
Furthermore, with the upsizing of a crystal diameter, the location of the melt surface is caused to change greatly by the weight of the quartz crucible (variations of wall thickness), deformation during operation, swelling and the like, thereby causing a problem that the location of the melt surface changes for every crystal growth batch. Therefore it is getting more and more difficult to control with precision the distance between the melt surface and the heat insulating member in such a manner that the predetermined distance is maintained.
For the improvement of these, for example, Japanese Patent Application Laid-Open Publication No. H6-116083 proposes placing a reference member in a CZ furnace and determining a relative distance between a real image of the reference member and a mirror image of the reference member reflected on the melt surface to determine the distance between the reference member and the melt surface. Then, based on a result of the determination, the distance between the melt surface and the heat insulating member is controlled with precision in such a manner that the predetermined distance is maintained.
Moreover, Japanese Patent Application Laid-Open Publication No. 2001-342095 discloses a method of taking into consideration the curvature of the raw material melt resulting from the rotation of the crucible in order to achieve the stability of the mirror image of the reference member.
In these methods, a picture is taken of a real image of the reference member and a mirror image of the reference member with a detector such as an optical camera and the brightness of the picture taken of the real image and the mirror image of the reference member is quantized into two output values (binarization) based on a certain predetermined threshold value (a threshold value of binarization level). In other words, a distinction is made depending on whether a spot is bright or dark compared with the threshold value of binarization level. Subsequently, measurement is made as to where the location of an edge thereof is and the values of the measurement are converted to determine the distance between the real image and the mirror image.
However, with the passage of time for a crystal growth process, the change in the brightness of the mirror image of the reference member reflected on the melt surface fluctuates a value detected with the optical camera before binarization or noise of a splash of melt resting on a structural unit in the CZ furnace and the like different from the mirror image of the reference member is detected, thereby causing a problem that the distance between the reference member and the melt surface is not determined stably or accurately.
Here, FIG. 3 are illustrations showing that it is impossible to determine a relative distance between the reference member and the melt surface accurately by a conventional method because the result of the determination changes. FIG. 3(a) shows a steady state and FIG. 3(b) shows a state where the brightness of the mirror image has fluctuated and increased. As is clear from FIG. 3, because the value detected with the optical camera before binarization fluctuates with the change in the brightness of the mirror image, accurate determination is not to be carried out by a conventional method.
On the other hand, for example, in the case of producing a silicon single crystal of 300 mm or more in diameter without applying a magnetic field from a raw material melt contained in a quartz crucible of 800 mm or more in bore diameter, there has also been a problem that, due to the vibration of the melt surface, the precise location of the melt surface is not to be detected stably. The relative distance between the reference member and the melt surface is not to be determined stably or accurately in this case, either.
When the determination result of the relative distance between the reference member and the melt surface is inaccurate, the distance between the melt surface and the heat insulating member is not to be controlled with precision in such a manner that the predetermined distance is maintained. Consequently, a silicon single crystal of desired quality is not to be efficiently produced.